Human immunodeficiency virus (HIV) is the etiological agent for acquired immune deficiency syndrome (AIDS). HIV infects cells of the immune system. In the course of the disease, the number of lymphocytes in the immune system becomes depleted, and the body is incapable of mounting a defense against other infections. Death eventually results as the immune system of an infected person becomes decreasingly effective in protecting the infected person.
HIV infects T-cells through CD4, a protein found on the cell membrane of those cells. The HIV protein gp120 binds to CD4 as the initial step in the infection of a T-cell by HIV. Upon infection, the virus replicates within the T-cell and eventually destroys the cell, yielding multiple virus particles which can then infect other T-cells. Thus, T-cells, cells which are part of the system that protects against infections, are the target of the infectious agent. At first, the immune system provides protection and disease development from initial HIV infection to AIDS is slow. As the infection progresses, the immune system become increasingly depleted and compromised. Finally, the immune system becomes incapable of adequately functioning and death results, usually by an opportunistic infection the body is incapable of defending against.
There is no cure for AIDS and the treatments currently available are ineffective at substantially lengthening life. Moreover, the therapy available to prevent people infected with HIV from developing AIDS is equally ineffective. Given the exponential growth of this pandemic, it is estimated that up to 40 million people will be infected with HIV by the year 2000. A treatment must be found that is capable of extremely large, rapid, inexpensive production or possibly every one of the estimated 40 million people will die prematurely. There is a critical need for a more effective treatment for people suffering from AIDS. Furthermore, there is a need for a more effective therapy for people who are HIV positive but do not have AIDS in order to prevent them from developing AIDS.
Both the government and the pharmaceutical industry have devoted vast resources to finding an effective treatment for people infected with HIV. The commitment of money and man-power has been significant in an effort to overcome this currently intractable problem which defies a solution. Enormous amounts of money have been earmarked and spent on research. Large numbers of scientists are actively engaged in AIDS research, and in particular, in drug discovery efforts to identify compounds and methods to stop or slow the destructive progress of the virus in infected people.
Currently, a great deal of effort is being made to discover compounds and methods that prevent or retard HIV replication and the destruction of the immune system in infected persons. Although some progress has been made, there have been no discoveries that vastly increase the life expectancy of infected persons.
One strategy is to interfere with viral enzymes essential for viral replication. There is a great deal of interest in finding compounds that inhibit HIV reverse transcriptase (RT) or HIV protease. Currently, some success has been made using RT inhibitors. However, it has been only limited success. Over time, RT inhibitors appear to lose their effectiveness.
Another approach to combatting the virus is to selectively destroy infected T-cells by targeting gp120 using CD4 conjugated to a toxin or antigen. The CD4-conjugate is supposed to bind to gp120 associated with infected cells. In the case of CD4-toxin conjugates, the toxin is then available to kill the cell and thereby prevent viral replication. If a CD4-antigen conjugate is used, the CD4-conjugate that is bound to the infected cell will provide a good target for removal by the immune system before viral replication is complete. The infected cell will be killed and its contents destroyed. Thus far, no successful treatment has been developed from these efforts.
Many other strategies and approaches have also been employed. However, despite the desperate need and the enormous drug discovery effort, no successful treatment has been developed to stop or effectively slow the progress of infection from initial viral infection to the eventual destruction of the immune system characteristic of AIDS.
C. D. Pauza, (1988) Cellular Immunol. 112:414-424, reports that infected monocytes act as reservoirs of HIV, and further are involved in presentation of HIV to T-cells. Pauza discusses the need to focus more attention on the effects that the progression of AIDS has on monocyte cells and the breakdown of natural immunity associated with HIV infection of them. Pauza relates that the abrogation of normal effectiveness of monocyte cells in natural immunity caused by HIV infection of monocytes results in the sensitivity an individual has to opportunistic infections. Rather than targeting monocyte cells, however, Pauza discusses the deleterious effects on the HIV-infected individual caused by the lack of functional monocytes. The link is drawn between the lack of normally functioning monocytes and fatality in HIV infected individuals.
Metzler, et al., (1990) Immunology Today 11(6):217-223 review the role of macrophage cells in HIV disease. A discussion of persistently infected cells in the pathological development of HIV infection is included. Metzler et al. disclose the need to control HIV infection and replication in macrophages, and neither teach nor suggest the elimination of the cells.
McGrath, M. S. et al., (April 1989) Proc. Natl. Acad. Sci. USA 86:2844-2848, describe a compound, GLQ223, which is reported to inhibit HIV replication in vitro in cultured lymphocytes and mononuclear phagocytes. GLQ223, a 26kD protein that is purified from plant material, is similar in function and structure to the A chain of Ricin. McGrath et al. acknowledged the need for targeting HIV in mononuclear phagocytes as necessary for effectively impeding the advancement of disease. However, GLQ223 is directed at eliminating viral replication in monocytes, not elimination of monocytes themselves.
U.S. Pat. No., 4,869,903 issued Sep. 26, 1989 to Lifson et al. relates to a method of selectively inhibiting HIV in T-lymphocytes and monocytes that have been infected with HIV. The method comprises exposing infected cells to a plant derived single chain ribosome inactivating protein and inhibiting HIV replication while not destroying infected cells.
U.S. Pat. No., 4,795,739 issued Jan. 3, 1989 to Lifson et al. relates to a method of inhibiting expression of HIV antigens in T-lymphocytes and monocyte/macrophage cells that are infected with HIV. A method of treating human subjects infected with HIV is also disclosed. The specification contains a discussion of the problem caused by persistently infected monocyte/macrophage cells in the development of the disease in infected individuals. Two compounds, trichosanthin (TCS) and momorcharin (MMC), both of which are plant derived proteins, are disclosed as effective agents to inhibit expression of viral antigens in monocytes/macrophage cells that are infected with HIV. To solve the problem posed by persistently infected monocytes/macrophage cell, TCS and MMC are used to inhibit viral replication and production of viral proteins. There is no suggestion of destroying monocytes/macrophage cells. The disclosure contains instructions on screening compounds for anti-HIV activity. The screening assays are directed at identifying compounds which inhibit expression of viral antigens HIV infected monocytes/macrophage cells. Ricin, Abrin and Modeccin are among the compounds which are suggested to be used in the screening assay used to determine whether or not compounds inhibit expression of viral antigens in monocytes/macrophage cells. Both intact and A-chain forms of these compounds are suggested for screening. The method disclosed is directed at inhibiting viral replication and production of viral proteins without loss of viability to infected cells. The aim of the invention is to inhibit viral replication while not harming the cells.
Ricin, also know as Ricin D and RCA-II, is a monovalent lectin derived from caster beans. Ricin, a toxin, is composed of two dissimilar peptide chains. The B-chain is responsible for binding to, and facilitating entrance into, the target cell. Upon entering the cell, the A-chain enzymatically inactivates the 60S ribosomal subunit, thereby inhibiting protein synthesis and causing cell death by apoptosis.
Simmons, B. M. et al., (1986) J. Biol. Chem. 261(17):7912-7920, disclose the role of the high mannose carbohydrate chains in the mechanism of action of Ricin toxin. The mannose receptor-mediated uptake of Ricin toxin and Ricin A chain by macrophages is discussed as well as the binding of the Ricin galactose receptor to cell surface glycoproteins. The role of carbohydrate in the multiple intracellular pathways for chain translocation were investigated.
Zenilman, M. E. et al., (1989) Transplantation 47(1):200-203, reports that intraportal administration of intact Ricin to mice resulted in the selective depletion of Kupffer cells. Zenilman et al. suggest that selective depletion of donor Kupffer cells prior to organ transplantation may decrease donor immunogenicity and therefore reduce rejection. Zenilman et al. report that intact Ricin is significantly more effective than Ricin A-chain alone in depletion of Kupffer cells.
For the past decade, immunotoxins have been formed using either Ricin A-chain or intact Ricin conjugated with a specific targeting antibody. The antibody is directed at a cell surface protein or a viral protein that is displayed on the surface of virally infected cells. When a cell ingests the Ricin-antibody complex bound to its surface, the cell is killed. Great efforts have been made to try and bypass the "natural tendency" for these immunotoxins to be removed from the bloodstream by macrophages in the spleen and liver. The greatest success at reducing this undesired macrophage toxicity has been through the elimination of Ricin's carbohydrate side chain.
In addition to Ricin, other ribosome-inactivating proteins include Abrin, Modeccin, Viscumin and Volkensin.
Abrin, another monovalent lectin, is a protein extracted from Abrus precatorius seeds. Abrin, similar to Ricin, is a toxin comprised of dissimilar peptide chains.
Modeccin, another toxic lectin, is also a ribosomal-inactivating protein derived from plants. Modeccin can be extracted from the root of Adenia digitata. Modeccin is also composed of two dissimilar peptide chains.
Viscumin is a toxic lectin that very similar to Ricin. Viscumin, which is also comprised of two dissimilar peptide chains, is extracted from Viscum album L. (mistletoe)
Volkensin, another toxic lectin, is also a ribosomal inactivating protein derived from plants. Volkensin can be extracted from Adenia volkensii. Volkensin is also composed of two dissimilar peptide chains.
The present invention provides a method of treating humans suffering from HIV infection by administering compounds which selectively eliminate MPL cells. According to the invention, MPL cells are selectively targeted for destruction in order to eliminate them as HIV reservoirs. By selectively destroying MPL cells, the advancement of HIV infection is disrupted by interfering with a mechanism of HIV proliferation in which the virus is maintained in the body and presented to T-cells, furthering the replication of the virus. According to the invention, HIV can be eradicated or the progress of the infection severely curtailed in an individual by eliminating the reservoir which enables HIV to persist during the body's effort to remove it.